Co-based Superalloys Research Articles

Investigation of microstructure and mechanical properties in dissimilar-metal welding between ferritic stainless steel and Co-based superalloy using electron beam welding

This research investigates the relationship between the microstructure and mechanical properties of electron beam welding joints, with a particular emphasis on the influence of varying electron beam currents on AISI 410L stainless steel plates and NS163 Co-based superalloy wires, which is a rare and novel form of welding. An increase in beam current led to a gradual transformation of the weld microstructure from columnar to equiaxed grains, creating finer and more homogeneous grains. Macrosegregation was also observed, a consequence of the liquidus temperature difference between the weld metal (WM) and base metal (BM). The segregation structure exhibited irregularly shaped beaches near the fusion line and islands of unmixed and partially mixed NS163 within the WM. This phenomenon was more pronounced in samples with a lower beam current. Given the limitations in BM shape and WM size, nanoindentation was employed to characterize the strength of welded joints. In conjunction with microhardness measurements, it was confirmed that the weld is strengthened under different electron beam currents, with higher current resulting in a more significant strengthening effect. This study also underscores the value of nanoindentation in evaluating the weld strength of brush seals without destruction.

Numerical Simulation on Electron Beam Smelting Temperature Field of Novel Ni–Co-Based Superalloy

Numerical Simulation on Electron Beam Smelting Temperature Field of Novel Ni–Co-Based Superalloy

Interface healing mechanism of fine-grained Ni–Co‐based superalloy during hot‐compression bonding

The interface healing mechanism of fine-grained Ni–Co-based superalloy during hot-compression bonding (HCB) is investigated. During HCB, the incompatibility of deformation between the γ and the primary γ′ leads to a large number of dislocation pairs (DP), stacking faults (SF), and micro-twins (MT) in the primary γ′. These defects act as fast channels for elemental diffusion, leading to supersaturation of the primary γ′ and promoting the growth of the γ-shell. On the one hand, the primary γ′ with a γ-shell moves towards the bonding interface due to anomalous yielding phenomena of the primary γ′ and plastic flow during HCB process. The increase in the number of defects leads to the growth of γ-γ′ heterogeneous epitaxial recrystallization (HERX) grain with coherent structure at the bonding interface, which promotes the bulge of the interface grain boundaries (IGBs). On the other hand, the nucleation and growth of a necklace-like distribution of discontinuous dynamic recrystallization (DDRX) grain at the interface lead to the healing of IGBs. With the synergistic action of DDRX and HERX, the mechanical properties of Ni–Co-based superalloy joints through HCB achieve the same level as the base material. This finding further enriches the theory of interface healing in HCB.

Investigation of the Layer Effects Formed by W-EDM on Electrochemical Grooving of Stellite 21

Machining hard-to-cut materials, such as cobalt (Co)-based superalloys, is a common problem in manufacturing industries. Background: wire electrical discharge machining (W-EDM) is one of the widely used cutting processes that causes layer (white layer—WL and heat-affected zone—HAZ) formation, and microcracks on the material’s surface. Purpose: this study investigates the effects of WL and HAZ on the electrochemical grooving (EC grooving) performance of Co-based superalloys. Two different surface types (W-EDMed and VFed) were used in the experiments. Result: the experiments showed that material removal rate (MRR) values increased up to 212.49% and 122.23% for vibratory finished (VFed) and wire-electrical-discharge-machined (W-EDMed) surfaces, respectively. Conclusion: This result indicates the presence of HAZ and WL that prevent current transition between two electrodes. However, increased voltage causes an increase in surface roughness, with increment rates at 71.13% and 36.08% for VFed and W-EDMed surfaces, respectively. Moreover, for the VFed surfaces, the groove lost its flatness at the bottom after an approximately 100 µm depth due to the different electrochemical machineabilities of HAZ and real surface texture. This result can be attributed to the different microstructures (HAZ and surface texture) showing different electrochemical dissolution rates. Therefore, high-depth distance HAZ and WL must be removed from the workpiece.

Influence of Al and Mo on microstructure and tribological behaviors of Co-based superalloy

In this study, three Co-based superalloys with different Al and Mo concentrations were prepared by powder metallurgy (PM) method to investigate their influences on the microstructure and tribological properties. Replacing Al with Mo in the alloy has been found to decrease its hardness and yield strength, but increases its compressive ductility at room temperature. With the increase of Mo content, the formation of the HCP phase from the original FCC solid solution phase was promoted, and the microstructure of the alloy became relatively coarse. All superalloys showed decreased friction coefficient during sliding wear at 800 °C. The alloy with the highest content of Mo and lowest content of Al exhibited the greatest wear resistance at both room temperature and 800 °C. Formation of a continuous compact glaze layer rich in Cr2O3 and CoMoO4 oxides results into a significant reduction in both friction coefficient and specific wear resistance, and the synergistic effect of high temperature wear-induced oxides is the dominant contribution rather than the transformation of HCP from the original FCC solid solution phase at high temperature.

Hot deformation characteristics and microstructure evolution of Al20Co36Cr4Fe4Ni36 eutectic high entropy alloy

Eutectic high entropy alloys (EHEAs) have garnered much research attention due to their excellent mechanical properties, superior castability, good oxidation resistance, and stable microstructure in a wide range of temperatures. Inspires from mature Ni and Co-based superalloys, a novel Al20Co36Cr4Fe4Ni36 EHEA was designed and prepared to obtain good mechanical properties at both room temperature and high temperatures in this study. The novel EHEA consists of a L12 phase and a B2 phase with a superior ultra-tensile strength of 1005 ± 40 MPa and a ductility of 8.8 ± 0.4% at room temperature. The engineering compressive stress–strain curves at 1073 K indicated that the current EHEA performs a high strength of 321 ± 9 MPa without fracture even at a high temperature of ~0.6 TE, which is even superior to some refractory high entropy alloys (RHEAs). The hot deformation behavior and microstructure evolution of the new EHEA was investigated at the temperature range of 1073 K to 1273 K and various strain rates from 10−3 s−1 to 10−1 s−1. The constitutive equation that describes the correlations between the flow stress, strain rate, and temperatures was constructed. Continuous dynamic recrystallization (CDRX) was the primary dynamic recrystallization (DRX) mechanism in both the L12 and B2 phases during hot deformation. However, the proportion of dynamic recrystallization grains in the L12 phase is higher than that in the B2 phase according to the high angle grain boundaries (HAGBs) under the same deformation condition, suggesting that the CDRX kinetics was faster and stronger in the L12 phase. The present study provided a new thought for designing EHEAs and an insight into the hot deformation behavior of EHEAs with lamellar morphologies.

Influence of the γ′ Volume Fraction on the High-Temperature Strength of Single Crystalline Co–Al–W–Ta Superalloys

Understanding the influence of γ′ and secondary-phase fractions on the mechanical properties of superalloys is very important to optimize these high-strength materials. So far, this has not been systematically investigated for the novel class of Co-based superalloys. In this study, a Co–Al–W–Ta model alloy series was designed with compositions of γ/γ′ on the tie-line and an increasing γ′ volume fraction of up to 70% after heat treatment at 900 °C, while a few alloys are unexpectedly out of γ/γ′ two-phase region with an additional secondary phase fraction of up to 15%. The high-temperature strength and creep properties were evaluated by compression tests up to 1050 °C and compressive creep experiments at 950 °C, respectively. At temperatures of up to 1050 °C, an increasing γ′ volume fraction consistently increased the yield strength, which was not dramatically changed by the presence of secondary phases. Significant work hardening was found in alloys with γ′ volume fractions of 65–70% during compression testing, but not in alloys with either a lower γ′ volume fraction (<50%) or a high fraction of secondary phases (~15%). Similar to the yield strength, the creep strength also increased continuously with the γ′ volume fraction, but was greatly reduced with an increasing fraction of secondary phases. The best creep performance at 950 °C and 200 MPa was found in the alloy with the highest γ′ volume fraction and no secondary phases. At higher creep stresses, rafting contributed significantly to the hardening and, again, the alloy with a high γ′ volume fraction and a small amount of secondary phases exhibited the highest strength.

Effect of strain rate and high temperature on quasi-static and dynamic compressive behavior of forged GH5188 superalloy

GH5188 Co-based superalloy is commonly employed in aero-engine eddy current and high-temperature components. Nonetheless, current research on its response behavior and mechanisms under dynamic and temperature loading remains scarce. Therefore, this study aims to investigate the dynamic compression and related deformation mechanism of GH5188 superalloy under the coupling effect of high temperature and strain rate. Quasi-static and dynamic compression tests were conducted on the forged GH5188 superalloy at both room temperature and 1000 °C. The results indicate that the material displays impressive yield strength and plasticity, while also showing a pronounced temperature sensitivity and strain rate hardening effect. Additionally, the dynamic stress-strain constitutive relationship of GH5188 is described by the modified J-C model. Regarding the dynamic deformation mechanism, GH5188 exhibited microbands, stacking faults, and nanotwins during high-strain rate compression. At 1000 °C, GH5188 underwent discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) at strain rates of 6000 s−1 and 10000 s−1, respectively. The synergistic effect of microbands and twinning enhanced the dynamic deformation ability and strengthening effect of the material. The transformation of microshear bands generated nanotwins. The twin boundaries are high-energy zones that can provide suitable recrystallization nucleation sites, induce dynamic recrystallization process, and thus obtain a fine and uniform equiaxed grain structure.

Creep property and rafting kinetics of Co-based monocrystal superalloys with antiphase boundaries of γʹ phase

The precipitation strengthened Co-based superalloys show the great potential for the next generation superalloys of aerospace engine, their mechanical properties are associated with the morphology stability of L12-γʹ phase under high temperature and loading state. The antiphase boundaries (APBs) formed by different variants of the ordered fcc-L12-γʹ-Co3(Al, W) phase, affect creep morphologies and the high-temperature properties of Co-based superalloys. By coupling with the Kim-Kim-Suzuki (KKS) model and crystal plasticity model, the directional rafting and evolution kinetics of the γʹ phase are studied during the creep of Co–10Al–10W (at.%) superalloy. Creep rafting under high temperature and low stress is a process of stress-induced directional coarsening, rafting is affected by the APBs on hindering the directional element diffusion. In the early stage of creep, the increasing number of APBs leads to a lower creep strain. Higher temperature accelerates the rafting and reduces the γʹ area fraction. Furthermore, under tensile stress and compressive stress, the APBs show the hindering effect on both P-type rafting and N-type rafting. This study reveals the effects of APBs on the rafting behaviors and creep properties, the findings are theoretically important for the microstructure optimization of Co-based superalloys.

Effect of Mo and V additions on the coarsening kinetics and high-temperature strength in γ′ precipitation-strengthened high-entropy alloys

High-entropy alloys (HEAs) containing well-stabilized nano-precipitates have a huge advantage in elevated temperature applications. This study investigated the effects of Mo and V additions on the morphological evolution, the coarsening kinetics of multi-component γ′ precipitates, and the high-temperature tensile strength of HEAs. HEAs with the additions of Mo and V have significantly lower lattice misfit values and exhibit a smaller coarsening rate of γ′ nanoparticles compared to Ni and Co-based superalloys. Moreover, the precipitate morphology related to the lattice misfit exhibits completely different effects attributed to Mo and V addition. Remarkably, Mo2.5V2.5 HEA has a yield strength of 496 MPa at 800 °C, which shows great potential for high-temperature applications.

Influence of Nb content on phase precipitation behavior of new Ni–Co-based superalloys by selective laser melting

In this paper, the evolution of the γ′ and η phases of new Ni–Co-based superalloy with different Nb contents was investigated using a combination of thermodynamic calculations and high-throughput selective laser melting (SLM). Thermodynamic calculations show that the increase of Nb content lowered the solubility temperature of the γ′ phase and caused the precipitation of the η phase. The alloys with the Nb gradient content prepared by SLM during aging at 800 °C for 100h indicated that the maximum volume fraction and size of the γ′ phase reach the maximum at 3.0–3.5% Nb. The elongated η phases enrichment in Ni, Ti, and Nb atoms precipitate for the Nb content higher than 2.2% Nb and prefer to grow longitudinally at the expense of the γ′ precipitates. Combining the influence of the Al/(Nb + Ta + Ti) ratio on the η precipitates, the addition of Nb content in the Ni–Co-based superalloys is less than 2.2%, which increases the high-temperature yield strength and tensile strength.

Active learning sensitivity analysis of γ'(L12) precipitate morphology of ternary co-based superalloys

To better understand the equilibrium γ′(L12) precipitate morphology in Co-based superalloys, a phase field modeling sensitivity analysis is conducted to examine how four phase-field parameters [initial Co concentration (c0), double-well barrier height (ω), gradient energy density coefficient (κ), and lattice misfit strain (ϵmisfit)] influence the γ′(L12) precipitate size and morphology. Gaussian Process Regression (GPR) models are used to fit the sample points and to generate surrogate models for both precipitate size and morphology. In an Active Learning approach, a Bayesian Optimization algorithm is coupled with the GPR models to suggest new sample points to calculate and efficiently update the models based on a reduction of uncertainty. The algorithm has a user-defined objective, which controls the balance between exploration and exploitation for new suggested points. Our methodology provides a qualitative and quantitative relationship between the γ′(L12) precipitate size and morphology and the four phase-field parameters, and concludes that the most sensitive phase-field parameter for precipitate size and morphology is the initial Co concentration (c0) and the double-well barrier height (ω), respectively. We note that the GPR model for precipitate morphology required adding a noise tolerance in order to avoid overfitting due to irregularities in some of the simulated equilibrium γ′(L12) precipitate morphology.

Alloying effects on site preference, mechanical properties, and deformation behavior of L12 Co–Ti-based alloys

Alloying plays an important role in controlling the phase stability, mechanical properties, and deformation behavior of ordered intermetallic compounds. In this study, the effects of 3d, 4d, and 5d transition elements on site preference, elastic properties, ideal shear strength, and planar fault energies of L12 Co–Ti-based alloys were systematically investigated by using first-principles calculations. The calculated transfer energy and formation enthalpy indicate that Sc, V, Cr, Y, Zr, Nb, Mo, W, Hf, Ta, and W tend to occupy the Ti site, which reduce the structural stability of L12-Co3Ti. The elastic moduli and ideal shear strength of L12-Co3(Ti,M) increase with average electron density. The electron localization function (ELF) analysis reveals that the Co–M bonds have a stronger covalent character than the Co–Ti bond, which plays an important role in the strengthening of the alloys. The ratio of superlattice intrinsic stacking fault (SISF) to anti-phase boundary (APB) energy of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period in the range of studied elements, which tends to change the deformation mode from the APB-favored to SISF-favored ones. The APB anisotropy ratio of L12-Co3(Ti,M) decreases with increasing atomic number of alloying elements in each period, which enhances the yield strength anomaly. This study not only sheds insight into the fundamental understanding of phase stability and mechanical behavior of multicomponent L12 compounds, but also provides useful guidelines for designing novel L12-stregnthened Co-based superalloys with superior mechanical properties.

A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property

Severe intermediate-temperature embrittlement as well as rapid softening and oxidation at high temperatures are significant problems for high-entropy alloys (HEAs) in polycrystalline structures, which greatly limit their applications in modern industries. In this study, a novel Co-rich single-crystal high-entropy alloy (HEA), Co41Ni35Al11·5Ta2·5Cr4Ti6 (at.%), was innovatively developed. After isothermally aged at 1250 °C, this alloy exhibits an ultrahigh volume fraction of cuboidal L12-type nanoprecipitates (∼80%) which is higher than those of previous Co-based superalloys. More importantly, the present HEA shows excellent mechanical performance at both ambient and elevated temperatures, and no intermediate temperature embrittlement and rapid softening were observed. Noticeably, the yield strength can still maintain at ∼800 and ∼500 MPa when tested at 900 and 1000 °C, which are superior to other conventional single-crystal alloys. Moreover, extraordinary oxidation resistance can be achieved at the high temperature of 1000 °C due to the formation of a compact sandwich-structured oxide layer.

Atomistic simulations of diffusion in γ′ -strengthened Co-based superalloys and its connection to selective alumina formation in early-stage oxidation

The suppressed diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ precipitates of Co-based superalloys is suspected to be one of the reasons for the selective formation of alumina during the early-stage oxidation above $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In order to scrutinize this assumption, the diffusion properties of Al, Co, and W in a prototype structure of the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase, $\mathrm{L}{1}_{2}\ensuremath{-}{\mathrm{Co}}_{3}(\mathrm{Al},\mathrm{W})$, are investigated in a combined density functional theory (DFT) and kinetic Monte Carlo (KMC) study. A particular challenge in simulating diffusion in this ordered phase is the representation and evaluation of numerous migration pathways on the $\mathrm{L}{1}_{2}$ sublattices. With specified diffusion models for the $\mathrm{L}{1}_{2}$ structure and random solid solution ($\ensuremath{\gamma}$ phase), our KMC results reveal that Al diffusivity in the $\ensuremath{\gamma}$ phase with the same stoichiometric composition is more than two orders of magnitude higher than in the $\mathrm{L}{1}_{2}$ ordered ${\ensuremath{\gamma}}^{\ensuremath{'}}$ phase at $900{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Furthermore, in our rapid thermal exposure experiments for a model Co-based superalloy, the formation of alumina nuclei is observed preferentially in the $\ensuremath{\gamma}$ channels while the Al content is comparable in both channels and precipitates before oxidation. Combining these insights from experiment and simulation, the sluggish diffusion of Al in ${\ensuremath{\gamma}}^{\ensuremath{'}}$ appears to be a key factor in the selective formation of alumina observed in these systems.

Stability and Physical Properties of the L12-γ′ Phase in the CoNiAlTi-System

There is a current interest in Co-based superalloys hardened by a L12-γ′ phase because Co has a higher melting point than Ni and is more resistant against sulfidation attack. However, the Co-Al-W system many of those γ′ hardened Co-based superalloys are based on, has a number of drawbacks. The γ′ phase Co3(Al,W) is not stable at high temperature, the density of the alloys is very high and the oxidation resistance is insufficient. Due to this, there is an ongoing interest to develop γ′-hardened Co-based superalloys based on other systems. Here, first principles calculations are presented to investigate the properties of the γ′ L12-(Co0.5,Ni0.5)3(Al0.5,Ti0.5) phase and related L12 structures. (Co0.5,Ni0.5)3(Al0.5,Ti0.5) exhibits a lower energy of formation than Co3Ti and Co3(Al0.5,W0.5). Nevertheless, Ni3(Al0.5,Ti0.5) has an even lower energy of formation which is further lowered if Ti is enriched on the second sublattice. This finding is supported by analyzing the electronic densities of states. Ni3(Al0.5,Ti0.5) and especially Ni3(Al0.25,Ti0.75) exhibit Fermi levels close to the gap between binding and antibinding states, which is an indicator for stability. In addition to the stability of the γ′-phase in dependence on Ni and Ti content, also the elastic properties were calculated. Ni3(Al0.25,Ti0.75) is less elastic anisotropic and has higher Young’s and shear modulus compared to Ni3(Al0.5,Ti0.5) and (Co0.5,Ni0.5)3(Al0.5,Ti0.5).

Full-field analysis of damage under complex thermomechanical loading

This study combines infrared thermography, digital image correlation and machine learning to measure respectively temperature, strain and damage fields at high temperature. This is applied to thermomechanical fatigue (TMF) testing in presence of severe gradient for typical out-of-phase loading condition. This type of loading is challenging when dealing with thin sheets due to buckling risk induced by high temperature compression. For a Co-based superalloy (Haynes 188), this study demonstrates full-field identification/validation of both behaviour and damage models. Thermal gradient model is the key point in this analysis. With the coupling of TMF measurements by machine learning and DIC, local fatigue micro-crack growth rate and localisation are assessed through jump in displacement. TMF finite element analysis, of loading and damage, validates the whole model framework.

Influence of Ni content on the oxidation behavior of alumina-forming Co-based oxide dispersion strengthened superalloys at 900 °C

This work systematically investigated the effect of Ni additions of 10, 20 and 30 at% on the oxidation behavior of oxide dispersion-strengthened (ODS) Co-20Cr-15Al (at%) superalloys at 900 °C in air. Alumina scale with a duplex structure consisting of outer θ-alumina and inner α-alumina was formed on all tested Co-based ODS superalloys. Ni addition reduced the oxidation rate by inhibiting the growth of the outer θ-alumina during initial oxidation and the inner α-alumina during prolonged exposure. Cr2O3 formation between the interface of the θ- and α-alumina layers was confirmed with the increase of Ni content.

Quantitative analysis and formation mechanisms of non-metallic inclusions in GH4068 alloy prepared by vacuum induction melting

An automatic scanning electron microscope (SEM) analysis system and a self-designed superalloy inclusion analysis module were utilized to characterize the inclusions in the GH4068 Ni–Co-based superalloy prepared by vacuum induction melting (VIM). The morphologies, compositions, and distribution of the inclusions as well as the relationship between the contents of oxygen/nitrogen and inclusion were analyzed. The results indicate that the inclusions in the GH4068 alloy mainly include oxides, carbides, nitrides, carbonitrides and complex inclusions. The oxides are mainly concentrated in the top center of the cast ingots, and their content is higher than that at the bottom and edge. Nitrides and complex inclusions are mainly distributed near the bottom and edge of ingots. The analyses of thermodynamics indicate that the homogeneous nucleation of Al2O3 is spontaneous during the solidification although TiN prefers to wrap on the nucleated particles rather than homogeneously precipitate from the melt. The increase of oxygen/nitrogen content results in the increase of oxides, nitrides and complex inclusions. However, when the concentration of nitrogen is in the range of 3–12 ppm, the increase of nitrogen content shows negligible influence on the content of nitride inclusions.

Effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy

In this work, the effect of cold rolling on microstructure, texture, and tensile properties of a Ni-Fe-based superalloy for Generation Ⅳ nuclear reactor application was systematically investigated. The results show that different deformed microstructures were observed during cold rolling, including dislocation tangles, slip bands, and microtwins (MTs). The dominant cold rolling deformation mechanism of the alloy gradually changed from planar slipping to deformation twinning, which was a little different from Ni-/Co-based superalloys. A texture transition from Copper {112}< 111 > to Brass {110}< 112 > and Goss {110}< 001 > at 25 % rolling reduction was detected due to the changed deformation mechanism. The standard heat treatment alloy exhibited a yield strength of 213 MPa and good elongation of 54 %. By increasing the rolling reduction up to 55 %, the yield strength was significantly enhanced to 872 MPa, which was attributed to the combined strengthening from both MTs and dislocations. Fine-spaced MTs with coherent interfaces can impede dislocation motion effectively and play a critical role in the strengthening, contributing 75–235 MPa to the yield strength. MTs can also provide ample room for dislocation accumulation, which maintained a reasonable elongation of 12 %. Moreover, due to the texture evolution during cold rolling, the averaged Schmid factor (SF) values for {111}< 110 > slip system exhibited a decreasing trend with the increased rolling reduction, contributing to the progressive increase in yield strength. The present work demonstrates that the formation of MTs at high rolling reductions is an essential reason for the improvement of mechanical properties of the alloy, which also provides a new strategy for designing and optimizing high-performance solid-solution strengthened Ni-Fe-based superalloys.